Method of producing cyanuric chloride

ABSTRACT

The invention relates to a method of producing cyanuric chloride by trimerizing chlorocyan at a temperature of at least 250° C. on washed activated carbon as the catalyst. The service life of the catalyst can be improved by using an activated coal with an effective pore volume V eff of equal or greater 0.17 ml/g, with V eff being the result of pores with a pore diameter ranging from 0.5 to 7 nm.

This invention relates to a process for producing cyanuric chloride bytrimerisation of cyanogen chloride at a temperature of above 200° C. onan activated carbon catalyst. The process according to the inventionalso results in a decreased specific catalyst consumption.

DISCUSSION OF THE BACKGROUND

Cyanuric chloride is produced on a large scale by chlorination ofhydrogen cyanide with the formation of cyanogen chloride andtrimerisation of the cyanogen chloride to form cyanuric chloride—seeUllmann's Encyclopedia of Industrial Chemistry Vol. A8, 5^(th) ed.(1987), 196-197. The trimerisation is carried out in the vapour phase ata temperature of above 200° C., in particular in the range of about 300to 450° C., on an activated carbon catalyst. During continuousoperation, a temperature profile develops along the longitudinal axis ofthe reactor owing to the exothermicity of the trimerisation reaction;this results in the formation of a so-called hot-spot, the temperaturemaximum of which depends on the flow rate and rises with increasing flowrate. It is known that the deactivation of the activated carbon catalystis influenced by the operating conditions, the flow rate and the qualityof the activated carbon. The deactivation becomes apparent from themovement of the reaction zone, and with it the temperature maximum,along the longitudinal axis of the catalyst.

Owing to its becoming deactivated, the catalyst has to be exchangedperiodically or otherwise activated. The economic efficiency of thecyanuric chloride process depends considerably on the service life ofthe catalyst, as not only the cost of the catalyst but also the cost ofa plant standstill have to be taken into account. Moreover, withincreasing deactivation of the catalyst, secondary products such as, forexample, cyameluric chloride, are increasingly discharged and hencenecessitate increased expenditure on the purification of the cyanuricchloride.

In view of the problems demonstrated, the experts have for a long timebeen interested in finding activated carbon catalysts which have anincreased service life and/or in varying the operating conditions insuch a way that the service life can be increased.

Accordingly, U.S. Pat. No. 3,312,697 discloses a process for producingcyanuric chloride using an activated carbon catalyst having a specificsurface of above 1000 m²/g, in which the activated carbon catalyst wasactivated by a treatment with acids and/or alkalies and a downstreamwashing with water. As a result of the above-mentioned treatment,inorganic constituents such as oxides, hydroxides and salts of metalssuch as Li, Mg, Ce, Ti, V, Mn, Fe, Ni, Pt, Cu, Zn, Cd, Sn, Pb and Bi,which diminish the service life of the catalyst, are dissolved out ofthe activated carbon. The service life of the catalyst is furtherincreased in this process by the addition of 0.5 to 10 wt. % chlorineand/or phosgene to the cyanogen chloride.

In the process according to U.S. Pat. No. 3,707,544, the service life isincreased by mixing the trimerisation reactor with a mixture of anactivated carbon and a solid diluent having little or no catalyticactivity. The disadvantage of this process is that the space-time yieldis diminished and the expense of disposing of the deactivated catalystis increased, above all if the diluent is a non-combustible material.

In the process described in U.S. Pat. No. 3,867,382, an untreatedactivated carbon produced from coconut shells is used instead of anacid-washed activated carbon. This activated carbon has an internalsurface area of 1200 to 1500 m²/g, a micropore volume of at least 0.7cm³/g and an ash content of below 4 wt. %. Owing to the vegetable originof the raw material used for this activated carbon, it has a low contentof heavy metals and an acid wash is rendered unnecessary. It cannot beinferred from this document how the micropores are defined, i. e.whether they comprise all the internal pores, or micropores havingprecisely defined limiting values for the pore diameters. A considerabledisadvantage of the activated carbon used in the examples is that thebulk density, and hence the quantity required based on the reactorvolume, is very high and thus diminishes the economic efficiency.

In J. Beijing Inst. Chem. Technol. 20 (1993) 1, 55-58, E. Wang et al.explain that several factors, namely, the ash content, the iron content,the specific surface and the pore-size distribution, have to be takeninto account when selecting the catalysts for the cyanogen chloridetrimerisation. The selection of a suitable activated carbon iscomplicated by the fact that these factors may mutually influence oneanother. It is to be concluded from this document that it isadvantageous to use a carbon which has as high a specific surface aspossible and therefore contains numerous small pores. The latter help toenable the reaction to proceed on a relatively large number of activecentres. From the diagrams of the pore-size distribution of twodifferent activated carbons, it is suggested that the pores should havea diameter in particular of less than 2 nm. However, no information canbe drawn from the document as to how the individual factors influencethe service life of the catalyst in a production plant designed forcontinuous operation.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to demonstrate animproved process for producing cyanuric chloride by trimerisation ofcyanogen chloride, the improvement consisting in a decreased specificcatalyst consumption. A further object is to demonstrate the criteriawhereby the person skilled in the art can select an activated carboncatalyst having an extended service life for this type of reaction.Other objects can be inferred from the following description of theprocess according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Specific catalyst consumption a in relation to the effectivepore volume

FIG. 2: Movement of the hot spot through the reactor

DETAILED DESCRIPTION OF THE INVENTION

A process for producing cyanuric chloride has been found, comprisingtrimerisation of cyanogen chloride in the presence of a washed activatedcarbon having a BET surface area of at least 1000 m²/g and an Fe content(calculated as Fe₂O₃) of less than 0.15 wt. % at a temperature of atleast 250° C., which is characterised in that an activated carbon havingan effective pore volume V_(eff) of equal to or greater than 0.17 ml/gis used, V_(eff) being obtained from pores having a pore diameter in therange of 0.5 to 7 nm. The subclaims are directed towards preferredembodiments of the process.

It was found that the trimerisation of cyanogen chloride proceedssatisfactorily only in those pores having pore diameters in the range of0.5 to 7 nm, in particular 0.5 to 5 nm; the pore volume of these poresare to be at least 0.17 ml/g. Although the pore distribution ofactivated carbons can differ very widely depending upon the conditionsof their production, the effective pore volumes V_(eff) necessary forthe reaction can be defined from the sum of a volume increment for themicropores having a pore diameter of <2 nm and a volume increment of themesopores having a pore diameter of 2 to 30 nm. The effective porevolume accordingly can be represented as a linear function:V_(eef)=a·V_(micro)+b·V_(meso). It was also found that the functionV_(eff)=0.25·0.50 V_(micro)+V_(meso) is a suitable selection criterionfor an effective activated carbon having a long service life. Thevolumes of the micro- and mesopores are determined as follows:

The micropore volume is determined from the nitrogen adsorption isothermat the temperature of liquid nitrogen by comparison with a standardisotherm using the t-plot process of De Boer (cf. De Boer et al. in J.of Colloid and Interface Science 21, 405-44 (1966)) in accordance withDIN 66135, Part 2 (Version of April 1998).

The mesopore volume and the pore distribution are determined from thenitrogen desorption isotherm of Barett, Joyner and Halenda in accordancewith DIN 66134 (February 1998). Prior to the measurement, the sampleused for the determination of V_(micro) and V_(meso) is treated for 1 hat 200° C. under vacuum (less than 1.3 Pa). The measurement is carriedout, for example, in an “ASAP 2400” instrument manufactured by the firmof Micromeritics, Norcross, Ga. (US). The definition of V_(meso)according to the invention includes only mesopores having a diameter of2 to 30 nm.

A particularly large increase in the service life of the activatedcarbon in this type of process is achieved if V_(eff) is at least 0.2ml/g. From an investigation of numerous different activated carbons, itwas found that a maximum value of the effective pore volume definedabove corresponds to a minimum value of the specific catalystconsumption. Both extremely mesoporous activated carbons and extremelymicroporous activated carbons have too low a pore volume in the middlepore range, that is, in the range between 0.5 and 5 nm, so that thespecific catalyst consumption is considerably higher than in thecatalysts to be used according to the invention.

Another feature of the activated carbons to be used according to theinvention is the specific surface(BET surface area), which is at least1000 m²/g, preferably at least 1200 m²/g. A high surface area isconsequently advantageous, but is not a criterion which allows aconclusion regarding the service life of the catalyst. Thus, differentactivated carbons having virtually identical specific surfaces exhibitvery large differences in their rates of deactivation.

In view of the negative influence of a high iron content on theactivated carbon, the iron content, calculated as Fe₂O₃, should be below0.15 wt. % and preferably around or below 0.1 wt. %. Although anunwashed activated carbon is also catalytically active, in the processaccording to the invention a washed, in particular an acid-washed,activated carbon is used, because washing is on the one hand a possibleway of decreasing the content of iron and of the other heavy metals andhence of minimising the formation of secondary products and, on theother hand, it increases the pore volume, which is important for thereaction. With regard to the minimisation of the specific catalystconsumption, it is moreover advantageous to use a carbon having a bulkdensity of equal to or less than 420 g/l. Where the activity of theactivated carbon catalyst is adequate and the effective pore volumeis >0.17 ml/g, preferably equal or >0.20 ml/g, it is advantageous thatthe bulk density be as low as possible. In such cases it is advisable touse an activated carbon having a bulk density of equal to or <420 g/l,preferably <390 g/cm³. FIG. 1, which summarises the results of numerousinvestigations —see Examples—clearly shows the unforeseen extent towhich the specific catalyst consumption a (kg catalyst per t ofunreacted cyanogen chloride) is dependent on the effective pore volumedefined according to the invention when a washed activated carbon havinga BET surface area of at least 1000 m²/g and an Fe content of less than0.15 wt. % (calculated as Fe₂O₃) is used. The specific catalystconsumption is low, in particular when both the rate of deactivation(the method of determination may be found in the Examples) and at thesame time the bulk density of the catalyst are as low as possible.

EXAMPLES

The investigations to determine the specific catalyst consumption in thereaction zone during the trimerisation of cyanogen chloride to formcyanuric chloride were carried out in a tubular reactor filled with theactivated carbon catalyst being examined. The tubular reactor was cooledby means of a heat-transfer medium; the temperature of the coolant wasmaintained at 280° C. The test reactor was connected parallel to anoperating reactor. The gaseous cyanuric chloride formed was condensedafter having left the reactor and the liquid product was converted intothe solid aggregate state by being sprayed into cooled chambers.

The ratio of the length of the reactor to the cross-section of thereactor was 39. During continuous operation, a temperature profiledeveloped along the longitudinal axis of the reactor. This profilecomprises a heating zone, a reaction zone and a cooling zone. Themaximum of the reaction zone, the temperature of which rises withincreasing flow rate, moves forward in the direction of the flow, withincreasing deactivation of the catalyst. The rate of deactivation(U_(deact)) was determined by constructing time-dependent temperatureprofiles from temperature-measuring points arranged along the reactor.

FIG. 2 shows that with increasing operating time, the hot-spot of thereaction zone moves through the complete set of measuring pointsarranged one behind the other. The actual determination of the rate ofdeactivation was commenced by a so-called preliminary deactivation ofthe catalyst—at that time, the “hot-spot” developed near to the inlet tothe reactor. The preliminary deactivation of the catalyst lasts forabout 12 hours at a flow rate of cyanogen chloride of 1.1 kg per hour.FIG. 2 shows a typical progression of the deactivation. The rate ofdeactivation in cm/t ClCN can be determined from the distance of thetemperature-measuring points and the average quantity of cyanogenchloride (measured from maximum to maximum). The specific catalystconsumption in the reaction zone can be determined from the rate ofdeactivation (V_(deact).), the reactor geometry (cross-sectional area F)and the bulk density ρ, in accordance with the following equation:${{\, a}\left\lbrack \frac{{kg}\quad{{cat}.}}{t\quad{ClCN}} \right\rbrack} = {u_{Deact} \cdot \left\lbrack \frac{cm}{t\quad{ClCN}} \right\rbrack \cdot {F\left\lbrack {cm}^{2} \right\rbrack} \cdot {\rho\left\lbrack \frac{kg}{m^{3}} \right\rbrack}}$

TABLE 1 Activated carbon catalysts used Ash Fe content Bulk Catalyst Rawcontent (as Fe₂O₃) density BET Pore volumes (cm³/g) (No.) material Wash(wt. %) (wt. %) (g/l) (m²/g) V_(micro) V_(meso) V_(off)*) C1 Peat + 1.670.00 403 1016 0.38 0.18 0.185 C2 Peat + 2.45 0.07 346 1453 0.63 0.110.213 C3 Hard + 2.24 0.03 410 1217 0.51 0.17 0.212 coal C4 Wood + 2.180.28 375 1523 0.64 0.09 0.205 C5 Pine − 8.01 0.16 406 1290 0.58 0.110.200 wood C6 Coconut + 0.42 0.00 373 1459 0.59 0.04 0.157 C7 Peat +2.46 0.07 434 1213 0.50 0.08 0.165 C8 Coconut + 1.66 0.01 430 1110 0.450.07 0.147 *)V_(eff) = 0.25 V_(micro) + 0.5 V_(meso)

Table 2 shows the rate of deactivation u and the specific catalystconsumption a in the reaction zone using the activated carbons given inTable 1, the flow rate of ClCN being 4.4 kg per hour in all the tests.

TABLE 2 Rate of deactivation V and specific catalyst consumption a inthe reaction zone u a Catalyst No. (cm/t ClCN) kg cat./t ClCN C1 29 1.05C2 21 0.65 C3 25 0.92 C4*) 35 1.18 C5*) 40 1.46 C6*) 35 1.18 C7*) 281.09 Temperature of the heat-transfer medium: 280° C. *)activated carboncatalyst not according to the invention

The tests show that the specific catalyst consumption in the reactionzone depends considerably on the effective pore volume and the bulkdensity of the catalyst. As a result of a decreased consumption ofcatalyst, not only is the cost of the catalyst decreased, but at thesame time the availability of the plant is increased owing to decreasedstandstill times and the economic efficiency of the process therebylikewise increased.

1. Process for producing cyanuric chloride, comprising trimerisation ofcyanogen chloride in the presence of a washed activated carbon having aBET surface area of at least 1000 m²/g and an Fe content of less than0.15 wt. %, calculated as Fe₂O₃ at a temperature of at least 250° C.,wherein an activated carbon having an effective pore volume V_(eff) ofequal to or greater than 0.17 ml/g is used, V_(eff) is obtained frompores having a pore diameter in the range of 0.5 to 7 nm.
 2. Processaccording to claim 1, wherein the effective pore volume V_(eff) of theactivated carbon is calculated from the sumV_(eff)=0.25V_(micro)+0.5V_(meso), V_(micro) represents pores having adiameter of less than 2 nm and V_(meso) represents pores having adiameter of 2 to 30 nm.
 3. Process according to claim 1 wherein V_(eff)of the activated carbon used is at least 0.2 ml/g.
 4. Process accordingto claim 1, wherein the activated carbon has a bulk density of equal toor less than 420 g/l.
 5. Process according to claim 1, wherein theactivated carbon has a BET surface area of at least 1200 m²/g andV_(eff) is at least 0.2 ml/g.